1. Field of the Invention
The present invention relates to corrosion inhibition. More particularly, the present invention relates to corrosion inhibition in closed aqueous cooling systems. Still more particularly, the present invention relates to a composition and method for controlling corrosion in closed aqueous cooling systems.
2. Description of the Prior Art
Closed aqueous cooling systems with which the present invention is concerned are used in a variety of industrial situations such as, for example, gas and diesel engines, gas turbines, electric transformers, chilled water systems for air cooling, and the like. Such closed systems are usually constructed of a variety of dissimilar metals including copper, aluminum, cast iron, mild steel, brass, and the like, which can result in galvanic corrosion of the system. Sodium chromate has been a widely used corrosion inhibitor for such systems as has sodium nitrite, the latter usually being buffered with sodium borate making it particularly suited for use in systems comprising ferrous materials. More recently, however, the use of chromate inhibitors has been severely limited by Federal and State regulations because of their toxicity. Nitrite-based inhibitors are subject to attack by nitrifying bacteria which oxidize them to nitrates with subsequent microbiological growth and loss of efficacy with respect to ferrous materials. Silicate, nitrate and molybdate-based inhibitors, tolyltriazole, and polymeric based-inhibitors have all proved to be effective to one degree or another toward specific metals but are usually used in conjunction with borates, phosphates and nitrites.
A more detailed consideration of closed aqueous cooling system corrosion inhibitors is given in "Drew Principles of Industrial Water Treatment", pp. 147-151, published by Drew Chemical Corporation. The effectiveness of various inhibitors and combinations of inhibitors in aluminum closed aqueous cooling systems may be found in a paper entitled "The Effectiveness of Automotive Engine Coolant Inhibitors for Aluminum" by R. R. Wiggle et al published in National Association of Corrosion Engineers, June 1981, pp. 13-18. An investigation of molybdate, phosphate and borate inhibitors used in conjunction with nitrate and tolyltriazole to protect aluminum cooling systems is reported in a paper entitled "Evaluation of Molybdate as an Inhibitor in Automotive Engine Coolants" by M. S. Vukasovich, et al, August 1983, pp. 25-33. U.S. Pat. Nos. 2,815,328 and 3,335,096 disclose the use of sodium borate and sodium nitrate inhibitors in closed cooling systems while the use of various polymers in combination with silicate, nitrite, nitrate, borate and mercaptobenzothiazole is disclosed in U.S. Pat. No. 3,948,792.